An electrorheological fluid is known as a fluid which is obtained by dispersing dispersed phase particles in a dispersion medium composed of an electrically insulating solution, and which has a rheological characteristic that changes from the Newtonian viscosity to the Bingham viscosity under an externally applied electric field. It is also generally known that an electrorheological fluid composition shows a so-called Winslow's effect that the viscosity thereof greatly increases and a large shear stress is induced under an externally applied electric field.
Since the Winslow's effect has a characteristic that it shows a shear stress which quickly changes in response to an externally applied electric field, the electrorheological fluid can be applied to a variety of driving devices such as a clutch, a brake, an engine mount, a damper, a valve, a shock absorber, an actuator, an inkjet using an electrorheological fluid, etc.
Examples of conventional electrorheological fluids include those obtained by dispersing dispersed phase particles such as cellulose, starch, silica gel, ion exchange resin, zeolite, etc., into an electrically insulating solution such as silicone oil, diphenyl chloride, transformer oil, etc.
However, the described electrorheological fluids present the problem that a great change in the shear stress cannot be achieved and also a desirable durability in response to the above change cannot be achieved. Therefore, in practice, the above listed electrorheological fluids do not show desirable characteristics in practical applications.
Recently, earnest researches have been made in pursuit of the electrorheological fluid that is of use in practical applications. Most researches are related to the dispersed phase particles because it is considered that the dispersed phase particles are the essential component which directly affect the Winslow's effect.
Examples of the electrorheological fluid having a newly proposed dispersed phase particle include: the fluid including a poly(acene-quinone) (see Japanese Unexamined Patent Publication No. 216202/1986 (Tokukaisho 61-216202)); the fluid including composite particles having a three layer structure wherein an electrically conductive thin layer and an insulating thin layer are formed on an organic solid particle in this order (see Japanese Unexamined Patent Publication No. 97694/1988 (Tokukaisho 63-97694)); the fluid including composite particles wherein electrically conductive particles such as carbon black are dispersed in a resin (see Japanese Unexamined Patent Publication No. 236291/1989 (Tokukaihei 1-236291)); the fluid including carbonaceous powders (EP-361106A1)); the fluid including polyaniline (EP-394005); the fluid including zeolite (U.S. Pat. No. 4,744,914); the fluid including a sulfonated polymer particles in which the number of sulfonic acid groups exceed the number of the aromatic rings (U.S. Pat. No. 5,326,489), etc.
Ahmed (U.S. Pat. No. 4,992,192) discloses electrorheological fluids wherein polymer particles modifying its surface are used as dispersed phase particles to be added to a dispersion medium (vehicle). The modified polymer particle is prepared by polymerizing a substantially hydrophilic monomer on a surface of a substantially hydrophobic polymer particle.
The above listed publications include disclosures regarding improvements in the dispersed phase. However, none of them include disclosures regarding the dispersion medium.
In fact, there aren't many disclosures related to the dispersion medium being an essential component of the electrorheological fluid, and most of the publications disclose techniques for preventing the settlement of the dispersed phase particles by making smaller the difference in specific gravity between the dispersion medium and the dispersed phase particles. Examples of electrorheological fluids, including newly proposed dispersion medium include: halogenated compound (w082/04442 and GB2153372), fluorosilicone oil (EP-284268) and phosphazene compounds (see Japanese Laid-Open patent publication No. 139597/1991 (Tokukaihei 3-139597).
The electrorheological fluid composition has not yet been practically applied to a device because it shows low durability of the shear stress and also of the current density, and also because the viscosity in the absence of the electric field is too high, etc.
For the dispersion medium for the electrorheological fluid, silicone oil is generally used because it has a desirable electric insulation, and also because those which have a low viscosity of several tens cP (centi Poise) are obtainable at a low price.
Therefore, the inventors of the present invention examined the electrorheological fluid using silicone oil as a dispersion medium, with respect to the durability of the shear stress generated under an applied electric field and also the durability of the current density, and discovered that sufficient durability of the shear stress and current density cannot be achieved.